GB930841A - Improvements relating to picture communication systems - Google Patents

Abstract

930,841. Television. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Jan. 5, 1961 [Jan. 6, 1960], No. 507/60. Drawings to Specification. Class 40 (3). In a television system a reduction in the bandwidth requirement of the transmission channel is effected by transmitting signals representing selected fields or selected frames (2 Î field = frame) and at a receiver deriving from such signals further signals representing interpolated approximations of the fields or frames not transmitted and combining both sets of signals to form a composite signal suitable for conventional reproduction. By transmitting the selected fields or frames at a rate such that the normal duration of the selected and unselected fields or frames is taken for the transmission a bandwidth reduction factor equal to the ratio of the normal transmission time of the selected and unselected fields or frames to that of a single field or frame is realized. In a preferred way of carrying out the invention the received low-rate selected field or frame signals are stored in a first cathode-ray tube store from which they are subsequently read-out (at normal scanning rates) to form the first field or frame of the composite signal and, simultaneously with the first storage operation, are also stored in duplicate in interlaced fashion in a second store from which the interpolated approximations to the " missing " fields or frames are derived by comparison of alternate lines of each field or corresponding lines of alternate frames. A linear form of interpolation may be effected but in areas involving sharp contours (e.g. transitions between extreme black and white) a process termed contour interpolation is employed. This process involves the arresting of the motion of the scanning beam which " reads " one of the two lines between which interpolation is required when a sharp contour is reached and, at this instant, the doubling of the scan velocity of the beam scanning the other line. This latter scan continues at doubled velocity until a sharp contour in such other line is reached at which instant the scanning is arrested and the first beam caused to scan at doubled velocity until it reaches a position corresponding to that of the second beam. Thereafter both beams scan their respective lines at normal velocity until other contours are reached. Instead of employing cathode-ray tube stores magnetic stores or delay line stores or combinations of both may be employed.